1. Field of the Invention
The present invention relates to a method of molding a fiber-reinforced resin laminate and to a laminated molding. Particularly, the invention relates to a method of molding a lightweight fiber-reinforced resin laminate having excellent rigidity and strength and improved surface condition, as well as to a laminated molding obtained by the same.
2. Description of the Related Art
Conventionally, because of excellent mechanical characteristics such as tensile strength, rigidity, and heat resistance, resin moldings reinforced with glass fibers or like fibers are widely applied to automobile parts, such as inner panel cores, bumper absorbers, door steps, roof racks, rear quarter panels, air-cleaner cases, various pillars, and glove compartment lids, as well as building and civil engineering members, such as outer wall panels, partition panels, and cable troughs.
Such fiber-reinforced resin moldings may be manufactured by the injection molding method in which a molten resin containing fibers is injected into the interior of a die. The injection molding method facilitates the manufacture of a molding having a complicated shape and can advantageously mass-produce moldings having the same shape, since a predetermined molding cycle can be repeated continuously.
When a fiber-reinforced resin molding is to be manufactured by injection molding, the action of increasing the amount of fibers in an attempt to improve the strength and rigidity of a molding tends to increase the weight of a manufactured molding. Accordingly, there is proposed the expansion injection molding method in which a foaming agent is mixedly added to a material resin in order to reduce the weight of a molding and the mixture is subsequently expanded and molded (Japanese Patent Application Laid-Open (kokai) No. 7-247679, etc.).
However, the expansion injection molding method has involved difficulty in obtaining a sufficient expansion ratio even when a foaming agent is used in a relatively large amount. Further, even when a sufficient expansion ratio is obtained, foaming impairs the appearance of a molding. Also, even though reinforcing fibers are contained, a molding sometimes fail to attain satisfactory mechanical characteristics such as strength, rigidity, and impact resistance due to a large hollow formed therein.
In order to obtain a molding having good appearance through expansion injection molding, the counter pressure molding method may conceivably be employed. According to the counter pressure molding method, a gas is injected into a die cavity so as to maintain the internal pressure of the die at a pressure higher than the foaming pressure of a foaming agent. The internal pressure built within the die cavity suppresses the foaming of a foaming agent contained in a molten resin. After the surface of the molten resin is cooled so as to form a skin layer thereon, the foaming agent is allowed to foam.
However, in the counter pressure molding method, after a molten resin is cooled to a certain extent, a foaming agent is allowed to foam; consequently, an expansion ratio is limited, and a relative large hollow or cavity is likely to be formed in a molding. Thus, a molding does not necessarily attain sufficient lightness and strength. Also, since gas facilities, dies having excellent airtightness, and the like are needed in order to increase the pressure of the die cavity, the cost of manufacture increases accordingly.
To solve the above problems, the applicants of the present invention has proposed molding methods which utilize a springback phenomenon which occurs during the molding of a resin containing reinforcing fibers (Japanese Patent Application Nos. 8-28841 and 8-298600).
According to these molding methods, in order to form a skin layer on a resin surface, a molten resin which contains reinforcing fibers longer than a certain length is injected into a die cavity whose capacity is smaller than the volume of an intended molding, or the injected molten resin is once compressed. After being thus filled with the resin, the die cavity is expanded to the volume of the intended molding so as to expand the resin through the utilization of a springback phenomenon. Thus, countless continuous pores are formed within the completed molding.
In this case, when, due to a relatively low content of reinforcing fibers, a resin does not expand sufficiently merely through the utilization of a springback phenomenon, a small amount of foaming agent is added so as to sufficiently expand the resin.
According to such a molding method, reinforcing fibers can be contained in an amount sufficient for obtaining a required strength, and countless pores are formed, in contrast with expansion molding which involves the formation of independent bubbles or a large hollow. Thus, a molding features both sufficient strength and sufficient lightness.
The aforementioned automobile parts and building and civil engineering members are formed from a laminate which is composed of a resin molding and a surface coat material attached onto the surface of the resin molding. Such a laminate has conventionally been manufactured by bonding a surface coat material onto the surface of a resin molding. However, because of a poor production efficiency of this method, there has been employed a method in which a surface coat material is united with a molding during molding so as to omit the step of bonding the surface coat material onto the surface of a molding. Specifically, a surface coat material is previously disposed within a die, a resin is charged into the die, and the charged resin undergoes molding to thereby unite the surface coat material and a molding.
In recent years, even in the case of a laminated molding which includes a surface coat material, there has been a strong demand for a reduction in weight. To meet the demand, a method for reducing the weight of a laminated molding through employment of press molding is proposed in Japanese Patent Application Laid-Open (kokai) No. 7-171841. The method comprises the steps of: disposing a surface coat material between male and female dies; charging a molten resin containing glass fibers and a foaming agent between the male or female die and the surface coat material; closing both dies; and opening the dies apart from each other so as to expand the molten resin through foaming.
However, according to the above press molding method disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-171841, since the dies are filled with the molten resin while they are open apart, burrs are likely to be formed. Further, since a gas generated by the foaming agent before the dies are closed leaks out from the dies, the expansion ratio of the resin decreases. Thus, a large amount of the foaming agent must be used in order to obtain a required expansion ratio. Also, a smooth skin layer is not formed on a surface portion of an obtained laminated molding not covered with the surface coat material. Since molding time is relatively long, manufacturing efficiency is impaired. Further, since the resin is expanded by opening the male and female dies apart from each other, a molding having a complicated shape, such as a ribbed molding, is difficult to manufacture.